Hydrogen is a next-generation energy source that is suitable for storage and transport, and has little environmental impact, and therefore hydrogen energy systems that use hydrogen as an energy carrier are attracting much interest. Currently, hydrogen is mainly produced by steam reforming or the like of fossil fuels, but from the viewpoints of problems such as global warming and fossil fuel depletion, the importance of alkaline water electrolysis using renewable energy as a power source continues to increase.
Water electrolysis can be broadly classified into two types. One type is alkaline water electrolysis, which uses a high-concentration alkaline aqueous solution as the electrolyte. The other type is solid polymer water electrolysis, which uses a solid polymer electrolyte (SPE) as the electrolyte. When large-scale hydrogen production is performed by water electrolysis, it is said that alkaline water electrolysis using an inexpensive material such as an iron-based metal of nickel or the like is more suitable than solid polymer water electrolysis using a diamond electrode or the like. The electrode reactions at the two electrodes are as follows.Anode reaction: 2OH−→H2O+½O2+2e−  (1)Cathode reaction: 2H2O+2e−→H2+2OH−  (2)
High-concentration alkaline aqueous solutions increase in conductivity as the temperature increases, but the corrosiveness also increases. Accordingly, the upper limit for the operating temperature is limited to about 80 to 90° C. The development of electrolyzer structural materials and various piping materials that are capable of withstanding higher temperatures and high-concentration alkaline aqueous solutions, and the development of low-resistance diaphragms and electrodes having increased surface area and provided with a catalyst have enabled electrolysis performance to be improved to about 1.7 to 1.9 V at a current density of 0.3 to 0.4 Acm−2 (efficiency: 78 to 87%).
A nickel-based material that is stable in high-concentration alkaline aqueous solutions is typically used as the anode for alkaline water electrolysis, and it has been reported that a Ni-based electrode has a lifespan of several decades or longer in alkaline water electrolysis that uses a stable power source (Non-Patent Documents 1 and 2). However, when renewable energy is used as the power source, degradation in the Ni anode performance caused by severe conditions such as abrupt start-stop operations and load fluctuations tends to be problematic (Non-Patent Document 3). The reason for this degradation is that it is known thermodynamically that nickel exists as a stable divalent hydroxide in alkaline aqueous solutions and that the oxidation reaction of the nickel metal proceeds near the potential of the oxygen generation reaction, and it is surmised that the type of nickel oxide production reaction outlined below proceeds.Ni+2OH−→Ni(OH)2+2e−  (3)
As the potential increases, oxidation to trivalent and tetravalent states occurs. The reaction formulas are as follows.Ni(OH)2+OH−→NiOOH+H2O+e−  (4)NiOOH+OH−→NiO2+H2O+e−  (5)
The nickel oxide production reaction and the reduction reaction of that oxide proceed at the metal surface, and therefore detachment of the electrode catalyst formed on the metal is accelerated. If the power required to perform the electrolysis can no longer be supplied, then electrolysis is halted, and the nickel anode is maintained at an electrode potential that is lower than the oxygen generation potential (1.23 V vs RHE) and higher than the potential of the cathode for hydrogen generation that functions as the counter electrode (0.00 V vs RHE). Electromotive forces generated by these chemical species occur inside the cell. The anode potential is maintained at a low potential due to progression of the cell reactions, in other words, oxide reduction reactions are promoted in accordance with the formulas (3), (4) and (5). In the case of an electrolyzer containing a combination of a plurality of cells, these types of cell reactions tend to cause current to leak through the piping connecting the cells, and therefore current prevention techniques are a matter that should always be considered. One such technique is a countermeasure in which a very small current flow is continued during stoppages, but this technique requires special power source control, and also results in continuous generation of oxygen and hydrogen, and therefore system control is time-consuming. In order to intentionally avoid a reverse current state, the above types of cell reactions can be prevented by removing the liquid immediately after a stoppage, but in the case where operations are performed using a power source such as renewable energy that is prone to large output fluctuations, this cannot be considered a suitable procedure.
In nickel-based cells, these types of oxides and hydroxides are used as active materials, but in alkaline water electrolysis, the activity of these types of nickel materials is preferably suppressed.
Conventionally, at least one component selected from among platinum-group metals, platinum-group metal oxides, valve metal oxides, iron-group oxides and lanthanide-group metal oxides has typically been used as the catalyst layer of the anode for oxygen generation that is used in alkaline water electrolysis. Other known anode catalysts include nickel-based alloys such as Ni—Co and Ni—Fe, surface area-expanded nickel, ceramic materials such as spinel Co3O4 and NiCo2O4, conductive oxides such as perovskite LaCoO3 and LaNiO3, noble metal oxides, and oxides formed from a lanthanide-group metal and a noble metal (Non-Patent Document 4).
In terms of the anode for oxygen generation used in alkaline water electrolysis, nickel itself has a small oxygen overvoltage, and sulfur-containing nickel-plated electrodes in particular are also used as the anode for water electrolysis.
An anode having a lithium-containing nickel oxide layer already formed on the surface of a nickel substrate is a known anode for oxygen generation for use in alkaline water electrolysis using a high-concentration alkaline aqueous solution (Patent Documents 1 and 2). An anode having a similar lithium-containing nickel oxide layer formed on the electrode has also been disclosed not for use in alkaline water electrolysis, but as a nickel electrode used in a hydrogen-oxygen fuel cell that uses an alkaline aqueous solution as the electrolyte (Patent Document 3). Patent Documents 1 to 3 include no mention of the lithium content relative to the nickel or the production conditions, and also make no mention of the stability of the electrode under conditions where the electric power suffers severe output fluctuations.
Patent Document 4 discloses an anode provided with a catalyst layer composed of a lithium-containing nickel oxide in which the molar ratio between lithium and nickel (Li/Ni) is within a range from 0.005 to 0.15. By using this catalyst layer, the crystal structure can be maintained even upon long-term use, and excellent corrosion resistance can also be maintained. As a result, the anode can be used in alkaline water electrolysis that uses a power source such as renewable energy that is prone to large output fluctuations.